The study of how afferent synaptic connections and activity influence neuronal development is central to further understanding of both neuroembryology and of the manner in which early sensory experience influences brain and behavior development. The proposed experiments will examine with light and electron microscopic methods the morphological development of two avian brain stem auditory nuclei--n. magnocellularis and n. laminaris. These experiments will contrast the normal ontogeny of these neurons with their development after embryonc deafferentation or early restriction of the acoustic environment. These experiments will bring together manipulation of morphological and experiential variables in order to examine a wide range of afferent influence in the early life of two functionally-related neuronal types with many advantages for such studies. A major objective of the proposed studies is to provide neuropathologic information on the central neural effects accompanying early acoustic deprivation or receptor damage. Several of the proposed experiments will involve early manipulations of the middle and inner ears or of early acoustic experience, with subsequent study of morphological development in second-order and third-order auditory neurons in the brain. These studies will provide a detailed and comprehensive survey of the effects on auditory neuron development of early conductive and sensorineural hearing loss.